Radio transmitter



Jan. 22, 1935.

E. G. PORTS 1,988,960

RADIO TRANSMITTER Filed Aug. 19, 1932 Mammy- "k 2 &-

INVENTOR EARL G. PORTS ATTORNEY Patented Jan. 22, 1935 UNITED STATES PATENT OFFICE RADIO TRANSMITTER Earl G. Ports, Livingston, N. J.,,assignor-to-Intenv national Communications Laboratories, Inc., j Newark, N. J a corporation of New York Application August 19, 1932, Serial No. 629,441 i 4Claims. (01. 250-17) The present invention relates to radio communication circuits and especially to circuits for radio transmission. More particularly, the invention comprehends; a" transmitter operable on I a plurality of wave bands, readily altered to operate on'any band and utilizing a minimum of equipment for such operation.

' The amplifying circuit used for highfrequency transmission is in itself novel. While this circuit is described hereinafter, in connection with this invention, its details are set'forth more fully andit is claimed in a co-pending application of Ci H. Suydam, Serial No. 624,942, whichis as signed to the assignee of this invention. v 15v One of the objects ofthis invention is to provide aradio transmitter adapted to transmit waves in a plurality of frequency bands and readily ad justed totransmit on a selected one of those bands. p p Another object of the invention is to provide transmitter circuits which diminish the tendency of the amplifying tubes 'to oscillate when high frequency waves are transmitted without increasing the bulk or cumbersomeness of the equipment when low frequencies are utilized.

A further object of the invention is to-provide means for easily and quickly adapting the transmitter for operation on a selected frequency band and for utilizing a desired frequency within that band.

Further objects will appear from the following specification and are set forth in detail in the annexed claims. 3

The invention will be more readily understood when the following specification isconsidered in connection with the attached drawing in' which the single figure shows the preferred form of my invention. 7 i

It is well known that the tendency of a vacuum tube, connected in the usual manner with the cathode grounded and the potential of the grid being varied with respect to the cathode, to self oscillate is greater at the higher frequencies. In order to reduce this tendency the circuitsfare arranged so that at high'frequency the amplifying tubeis connected inaccordance with the disclosure of the co-pending application mentioned above. This connectionfis hereinafter in the specification and claims termed the inverted connection. The inverted connection consists essentially of an input connected across the grid and cathode of an amplifying vacuum tube, an outputconnected across the grid and plate and a radio frequency ground connection to the grid, the potential of the cathode being varied with respect to the grid. Thus the inter electrode capacitance coupling the input and output circuits is that between plate and cathode, which is much smaller-than the capacitance coupling the input and output circuits'of 'a conventionallyconnected amplifier, namely that between plate and grid.

The inverted connection utilizes radio frequency choke coils to isolatethe cathode 'of theamplifying tube fromground. These high -fre-'-' quency choke coils aresmall and do not materially affect the size of the transmitter, and of course, as usual, offer very little impedance'to low frequency oscillations. However, choke coils, to be effective to isolate the grid'from ground at low frequencies, must be quite large and for this reason,'as well as because the advantages ofthe arrangement wherein the grid'is groundedat high frequencyare not so great when low frequencies are used, this circuit is not utilized for transmission on the low frequency band. When operation over the low frequency band is desired, switches are positioned to place the amplifying tube in a-conventionalamplifier circuit. Referring now to the drawing, an'oscillator tube, of the well known three-element type, designated 1f generates oscillations-which are then fed into an amplifying tube 2 from the output of which signals are fed mm the antenna 3 or the alternative output circuit 3Af y Assuming now that lowfrequency transmission is desired, switches 4,- 5, 6 and '7 are moved to their respective upper contacts. Thecircuits thus completed and the operation thereof are described'below. 1 Theplate or anode of tube 1 is connected through resistance 21, switch 5, tuning induce tance 10 (in parallel with which is condenser 11 switch 50 and direct current blocking condenser 40 to the cathode of tube 1. The cathode of tube 2 is now grounded at low frequency, for the high 40 frequency choke coils 48 and 49 offer very little, if any impedance to low frequency oscillations, Direct current potential is supplied to the anode from generator 30 through key 27, radio frequencyjchoke'coil'22, inductance 10 and voltage 45' lowering resistance 21. Radio frequency current flowing in 'the' plate circuit'and through primary coil 10 induces similar radio frequency currents in secondary inductance coils 12 'and 1 3.

Current induced in adjustable inductance 12 is fed back to the grid cathode circuit of tubeil and sustained oscillations of a desired frequencylre-g sult. Current induced in inductance 13 from the primary coil 10acting as asource of low fre: quency oscillations-is effective to produce potential variations on the grid of tube 2 and this potential variation produces amplified current in the plate circuit of tube 2, and, since that circuit includes primary coil 17, to which coil 23 in the antenna circuit is coupled, produces radio frequency current in the antenna. Plate voltage is supplied to tube 2 from generator 30 through radio frequency choke coil 31. As was the case with tube 1, a blocking condenser, designated 20, is placed in the plate circuit of tube 2 to isolate the anode voltage supply from ground 16.

It is to be noted that the arms of switches 50, 52 and 25 can be connected by a common oper-- ating member (not shown) and that, in like manner, the arms of switches 4, 5, 6 and 7 can be connected by a common operating member (not shown). When switches 4, 5, 6 and '7 are operated, the transmitter is conditioned for op-' eration on high or low frequencies, as may be desirable, while, when switches 50, 52, and 25 are operated, all tunable circuits are conditioned to produce a desired frequency within the selected band. In either case a slight variation in output frequency may be secured through operation of variometer 24 or of similar means not shown, in output circuit 3A.

Operation when the transmitter is conditioned for high frequency radiation will now be described. Switches 4, 5, 6 and- 7 are operated to place the arms thereof in their lower positions. The plate tuning circuit of vacuum tube 1 now comprises inductance 43 and variable condenser 44. I The energy feed back to sustain oscillations need no longer be external of the tube since the internal inter-electrode capacitance produces at high frequency sufficient reactive feed back to maintain oscillation. The grid circuit contains adjustable inductance, 45 to secure the proper grid voltage for maintaining oscillation.

The plate or output circuit of tube 1 is now coupled through adjustable condenser 46 to the cathode of tube 2. Inductance 43 of the high frequency oscillation circuit, to which variable coupling condenser 46 is connected, serves as a source of high frequency oscillations for the amplifier tube 2. The grid of tube 2 is now connected through condenser 47 to ground, the condenser being a low impedance path as concerns high frequency and a high impedance as concerns low frequency currents. The inter-electrode capacity coupling the input and output circuits of tube 2 is that between the cathode and-plate, and, since this is very low, oscillations are not produced in the amplifying tube even at these high .frequencies. The plate circuit isnow tuned by coil 18 and associated parallel connected condenser 19. Inorder to isolate the filament heating source 42.-f rom the input of tube 2, radio frequency choke coils 48 and 49 are provided. It is to be noted that, while the filaments of tubes 1 and 2 are shown connected in series, this is no part of the invention and a parallel connection would serve as well. Y

The plate circuit of tube v2, as before mentioned, contains coil 18 which is coupled magnetically to output circuit 3A. This output circuit is, of course, connected through appropriate means to a transmitting antenna, not shown. It is obvious that antenna 3 might also be used for high frequency transmission, if desired.

The meters 60, 61, 62 and .26 are for measuring current in, respectively, the grid circuit of tube 1, the grid circuit of tube 2 when in the low frequency connection, the grid circuit of tube 2 when in the high frequency connection, and the current in the antenna circuit. In practice, a single meter would be arranged to be switched into the circuit of either of the meters 60, 61 or 62.

From the above description, given for purposes of disclosure only and not to limit the scope of the appended claims, it is seen that a transmitter is here provided for radiating eitherv high or low waves and in which the tubes operate under favorable conditions during either set of connections.

It is understood that the circuits are applicable also to receivers and that the description is given asapplied to transmitters for simplicity only, similar receiver connections being within the scope of the invention.

What is claimed is:

J 1. In radio apparatus, the combination of a thermionic amplifying tube having cathode, grid and anode, sources of low and high frequency oscillations to be amplified by said tube, means to connect either source across'the cathode and grid, a switch, means effective upon actuation of the switch to one position to ground said grid at high frequency whilst said high frequency source is connected to said tube, and means effective upon actuation of said switch to another position to ground said cathode and isolate said grid from ground at low frequency whilst said low frequency source is connected to said tube.

2. In radio apparatus, the combination of a thermionic amplifying tube having cathode, grid and anode, sources of low and high frequency oscillations to be amplified by said tube, means to connect either source across the cathode and grid, circuit connecting means including a switch for maintaining the amplifier cathode at a fixed alternating current potential in relation to ground when low frequency oscillations are being fed to the amplifier to vary the potential of the grid and for maintaining the amplifier grid at a fixed alternating current potential relative to ground when high frequency oscillations are being fed to the amplifier to vary the potential of the cathode.

3. In a radio apparatus for operating in both high and low frequency ranges, the combination of a vacuum tube having cathode, grid and anode electrodes, a frequency determining circuit for the high frequency range, a frequency determining circuit for the low frequency range, circuit .connections including a switching means whereby in one position of the latter the high frequency determining circuit is connected to the electrodes of a tube which thereby produces high frequency oscillations and in another position the low frequency determining circuit is connected to the electrodes of .a tube which thereby produces low frequency oscillations, an amplifier having cathode, grid and anode electrodes, means for coupling the cathode anode circuit of the first tube to the cathode grid circuit of the second tube, and means including a two-position switch connected to the grid electrode of the amplifier and cooperating with said circuit connections and said coupling means whereby when high frequency oscillations are fed to the amplifier a high frequency ground is maintained on the grid electrode of the amplifier and potential of the cathode is varied with respect to the grid, and when low frequency oscillations are fed to the amplifier a low frequency ground is maintained on the cathode electrode of the amplifier and the potential of the grid is varied with respect to the cathode.

4. In a radio apparatus for operating in both high and low frequency ranges, the combination of a vacuum tube having cathode, grid and anode electrodes, a frequency determining circuit for the high frequency range, a frequency determining circuit for the low frequency range, circuit connections including a switching means whereby in one position of the latter the high frequency determining circuit is connected to the electrodes of said tube which thereby produces high frequency oscillations and in another position the low frequency determining circuit is connected to the electrodes of said tube which thereby produces low frequency oscillations, an amplifier having cathode, grid and anode electrodes, means for coupling the cathode anode circuit of the first tube to the cathode grid circuit of the second tube, circuit connections whereby when low frequency oscillations are being fed to the amplifier the potential of the amplifier grid is caused to vary relative to the potential of the amplifier cathode, and circuit connections by which a high potential point of said high frequency determining circuit is connected to the cathode electrode of the amplifier when high frequency oscillations are being fed to the ampli- 10 fier.

EARL G. PORTS. 

